Sorption of Eu3+ on Na-montmorillonite studied by time-resolved laser fluorescence spectroscopy and surface complexation modeling

The influence of pH and concentrations of Eu³⁺ and NaNO₃ on the sorption of Eu³⁺ to Na-montmorillonite were investigated through batch sorption measurements and time-resolved laser fluorescence spectroscopy. The pH had little effect on the distribution coefficients (K_d) in the range of pH 4–7 at 0.01 M NaNO₃, which indicates that the cation exchange reaction is a dominant sorption process. Meanwhile, the K_d strongly depended on pH at 1 M NaNO₃, suggesting the formation of inner-sphere surface complexes. A cation exchange model combined with a one-site non-electrostatic surface complexation model was successfully applied to the measured K_d data. Linear free-energy relationship was used to estimate the formation constants of the surface species from those of the corresponding aqueous hydrolyzed species. The TRLFS spectra of Eu³⁺ sorbed on Na-montmorillonite were processed by parallel factor analysis, which provided the fluorescence spectra, decay lifetimes, and relative intensity profiles of three Eu³⁺ surface species. These species corresponded to one outer-sphere (factor A) and two inner-sphere (factors B and C) complexes. It turned out that factors A and B correspond to Eu³⁺ sorbed by ion exchange to permanent charge sites of Na-montmorillonite and inner-sphere complexation with surface hydroxyl groups of the edge faces. Factor C became dominant at relatively high pH and ionic strength and likely corresponded to the precipitation of Eu(OH)₃ on the surface of Na-montmorillonite.